Information retrieval system with a tracking error threshold compensation of retrieved data and tracking error signals

ABSTRACT

As an optical spot (20) is scanned along a recording track (B) it interacts with optical markers (14, 16) disposed at regular intervals along adjacent sides of the track. Each interaction with an optical marker causes a peak value which is sampled and held (74, 76). A clock (46) derives a clock signal from the regular periodicity of the optical markers. A logic circuit (48) controls the sample and hold circuits such that the peak amplitude attributable to each optical marker is temporarily held. An adder (52) combines peak values attributable to optical markers on opposite sides of the track to produce a threshold signal (100) indicative of a sum or average thereof. A comparator (58) compares data (56) read from the track with the threshold signal. The comparator indicates a &#34;1&#34; level in response to the data signal exceeding a threshold a &#34;0&#34; level in response to the data signal being below the threshold level. A decoder (60) is clocked by the clock pulses to convert the 1 and 0 indications into a binary pulse train. In this manner, the threshold level which distinguishes between 1&#39;s and 0&#39;s varies or floats in accordance with the relative interaction of the optical beam and the optical markers on one or the other side of the track, i.e. the misalignment of the optical beam and the track. The threshold signal also varies in accordance with variations in the recording medium.

The present invention relates to the field of information retrieval. Itfinds particular application in conjunction with optical reproducingsystem for reading out data signals accurately and will be describedwith particular reference thereto. It is to be appreciated, however,that the invention may find application in conjunction with other dataretrieval apparatae.

BACKGROUND OF THE INVENTION

Heretofore, data has been electromagnetically recorded along data trackson a rotating disk. Optically monitorable structures or pits werearranged along the disk to refine the position of the tracks. Morespecifically, the tracks were arranged in concentric circles or in aspiral around an axis of rotation of the disk. Conventionally, the pitswere disposed to either side of the track. A concentrated laserlightspot was passed along the track, between the pits, and theintensity and polarity of reflective light were monitored. As the laserpassed over a pit, the intensity of reflected light was changed.

In perfect alignment, the reflected light intensity reduction as a spotpassed a pit on one side of the track was the same as the reflectedlight intensity reduction when the spot passed a pit on the other sideof the track. When the pits on one side of the track reduced theintensity of the reflected light more than pits on the other side, amisalignment was indicated. By monitoring the difference in theintensity of light reflected as a spot passed the pits on the left sideof the track relative to the intensity of reflected light as the spotpassed pits on the right side of the track, the tracking error could bedetermined. See for example U.S. Pat. No. 4,562,564 to Bircot et al.,issued Dec. 31, 1985. A servo system adjusted the position of thelightspot and any associated reading structure or readhead relative tothe rotating disk in accordance with the monitored left-right pitintensity difference.

Although laser light tracking systems were effective for maintainingtracking alignment, small tracking errors were inherent in the servocontrol and correction system. Even small tracking errors as small as0.1 micrometers caused significant variations in the amplitude of theread data signal. As the amplitude was reduced, interpretation of thedata signal became ambiguous. For example, when reading digital data,the amplitude of the analog value representing 1's was reduced. Withsufficient misalignment, the analog amplitude of a digital 1 was reducedinto the amplitude region allocated for 0's causing an encoded 1 to beread as a 0. Reading digital 1's as 0's, of course, caused significanterrors in the retrieved data.

In accordance with the present invention, a method and apparatus isprovided for reducing ambiguities in the retrieved data attributable toreadhead/track misalignment.

SUMMARY OF THE INVENTION

In accordance with the present invention, a degree of misalignment of alaser spot and a recording track is determined from the relativereflection or absorption of light by the optical markers on either sideof the track. An adjustment or compensation is made in the data signalin accordance with the determined degree of misalignment. Morespecifically to the preferred embodiment, the retrieved data signal iscompared with a reflective light intensity signal whose magnitude variesin accordance with the degree of determined misalignment. When readingdigital data, a threshold amplitude which demarcates between 1's and 0'svaries in accordance with the reflective light intensity signal andmedia characteristic deviation.

In accordance with another aspect of the present invention, an apparatusis provided for retrieving data that is encoded on a recording track. Areflective light monitoring means monitors reflectivity variations alongthe recording track. A threshold means set a threshold level inaccordance with the monitored reflectivity variations. A comparing meanscompares the threshold level with data read from the recording track.More specifically to the preferred embodiment, each read analog datavalue is compared with the threshold to determine whether it representsa 1 or a 0.

In accordance with an analogous aspect of the invention, a method isprovided for retrieving data encoded on a recording track which isscanned by a data reading means. Encoded data from the recording trackis read as the degree of misalignment between the recording track andthe reading means is monitored. A threshold level is adjusted inaccordance with monitored degree of misalignment. The read encoded datais interpretted in accordance with threshold level, which thresholdlevel varies with the degree of misalignment.

In accordance with a more limited aspect of the present invention, anapparatus for retrieving recorded data is provided. A recording mediumhas at least first and second optical reference markers, such as wobblepits, disposed adjacent a recording track. An optical scanning meansscans the recording track with an optical beam. A data reading meansretrieves data recorded on the track between the optical referencemarkers and produces an analog data signal in accordance therewith. Areflective light intensity signal means generates a reflective lightintensity signal which varies in accordance with a degree of interactionbetween the optical beam and the optical markers. A first sampling meanssamples the reflective light intensity signal as the beam scans adjacentthe first optical markers to produce first sampled signals. A secondsampling means samples the reflective light intensity signal as theoptical beam scans adjacent the second optical markers to produce asecond sampled signal. A combining means combines the first and secondsampled signals. A comparing means compares the combined first andsecond sampled signals with the retrieved analog data signal.

In accordance with yet another aspect of the present invention, a methodis provided for retrieving data encoded on a recording track which isscanned by an optical beam and which has first and second opticallyrecognizable markers therealong. The optical beam is scanned along thetrack. Variations are monitored as the beam is scanned adjacent thefirst and second optical reference markers. A threshold level isadjusted in accordance with the monitored variations. The encoded datais read from the track and compared with the threshold level.

A first advantage of the present invention is that it approves thereliability of retrieved data.

Another advantage of the present invention is that it improves thedifferentiation between 1's and 0's of digitally recorded data.

Another advantage of the present invention is that it reduces theinfluence of variations in media characteristics.

Yet another advantage of the present invention resides in an improvementin tracking stability of automatic tracking adjustment systems.

Further advantages of the present invention will become apparent tothose of the ordinary skill in the art upon reading and understandingthe following detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various steps and arrangements of stepsand in various components and arrangements of components. The drawingsare only for purposes of illustrating a preferred embodiment and are notto be construed as limiting the invention.

FIG. 1A illustrates a segment of a recording medium which is scanned bya laser lightspot in accordance with the present invention;

FIG. 1B illustrates reflected light intensity when the laser lightspotis centered on the data track;

FIG. 1C illustrates reflected light intensity when the center of thelaser lightspot is offset above the data track;

FIG. 1D illustrates reflected light intensity when the laser spot iscentered below the data track;

FIG. 2 is a diagrammatic illustration of an automatic data correctionsystem in accordance with the present invention;

FIG. 3 is a diagrammatic illustration of the output signals of likenumbered components of the circuit of FIG. 2;

FIG. 4 illustrates output signals of selected components of an alternateembodiment of FIG. 2 which encorpates signal smoothing low pass filters.

FIGS. 5A, B, and C illustrate alternate embodiments of the recordingmedium; and,

FIG. 6 illustrates an alternate embodiment in which the data signalteamed by differential detection.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1, data 10 is encoded, preferablyelectromagnetically, in data regions 12 centered along a virtual datarecording track B. A series of first optical reference markers 14 and aseries of second optical reference markers 16 are disposed at regularintervals symmetrically to opposite sides of the virtual track B.Preferably, the optical markers are wobble pits that are deviated orwobbled symmetrically, the same distance the left and right sides of thetrack. An adjacent track includes data segments 12', and analogouswobble pits 14' and 16'.

A focused laser lightspot 20 is disposed in alignment with an opticalsensor 22 which receives reflected light as the lightspot scans alongthe track. The amount or amplitude 24 of received light varies generallywith the degree of interaction between the lightspot and the markers. Inthe preferred embodiment, the amount of reflected light decreases as thelightspot interacts with each of the optical markers or wobbled pits.

Reference to FIG. 1B, when the lightspot is accurately aligned with thetrack B, peaks 26 and 28 corresponding to the left and right opticalmarkers 14 and 16, respectively, have the same amplitude or peak value.With reference to FIG. 1C, if the center of the lightspot 20, deviatestowards the left side of the track B and scans along a closely adjacentparallel path C, the lightspot interacts to a different degree with theleft and right optical markers. The peaks 26 from the left or firstoptical markers 14 with which the lightspot interacts more strongly thanwhen it is centered on track B are relatively large amplitude. Bydistinction, the peaks 28 from the right or second optical markers withwhich the lightspot interacts more weakly are smaller. A difference 30between the first marker peaks 26 and the second marker peaks 28 isindicative of the degree of misalignment between the optical spot 20 andthe virtual track B. With reference to FIG. 1D, when the optical spot iscentered to the right of the virtual track along a path D, the opticalspot interacts more strongly with the right or second optical markers 16than with the left or first optical markers 14. The first optical markerpeaks 26 are, accordingly, smaller than the second optical marker peaks28. The difference 30, in the peak values is again indicative of thedegree of misalignment. The sign of the difference 30 is indicative ofthe direction of the misalignment. The intensity or amplitude of theread data signal varies analogously to the peaks 26 and 28, although thevariation for a given amount of misalignment is preferably smaller thanthe difference 30.

When the lightspot 20 is centered on the track B, the peak values can besampled, held, and used as a reference value or basis for datarecognition. However, even a small deviation of the lightspot from thevirtual track B causes large variations in the peak values, renderingthe peak values unreliable as a reference values. The variations in thepeak values can become so large that a threshold or reference valuebased on the peak can vary to such an extent that data recognitionbecomes difficult. Because the wobble pits 14. 16 are disposed moreoff-center than the data 10 from the track B, misalignment of thelightspot and the track causes greater variations in the reflected lightfrom the pits or marks than in the sensed data.

In order to select a threshold or reference value for data recognitionwhich varies in accordance with the variations in a sensed or read datasignal, the first and second peak values 26, 28 are summed or averaged.The average level does not change as greatly as the difference betweenthe peak values. Rather, the average peak value varies more analogouslyto the variations in the sensed data amplitude.

With reference to FIG. 2, a misalignment monitoring means 40 monitorsmisalignment between the track and the optical spot 20 or other datareading means. More specifically, a reflective light intensity signalmeans 42 generates a reflectivity or light intensity signal that changesas the optical spot or beam scans the optical markers. A peak samplingmeans 44 samples the reflectivity or light intensity signal as the spotinteracts with each marker. A clock means 46 generates a clock signalfrom the periodicity of the markers. A logic circuit 48 controls thesampling and resetting of the peak sampling such that each left markerpeak value 26 and right marker peak value 28 is monitored. A thresholdmeans 50 sets a threshold in accordance with the misalignment monitoredby the misalignment monitoring means. More specifically, a summing oraveraging means 52 sums or averages the left and right marker peakvalues from the peak sampling means 44. A gain or amplitude adjustmentmeans 54 adjusts the relative magnitude of at least one at the averagepeak amplitude or threshold signal and a data signal from a data signalmeans 56. A comparing means 58 compares the threshold level with thedata signal. More specifically to the preferred embodiment, the datasignal is an analog signal which is indicative of digital 1's and 0's.The comparing means 56 compares the analog data signal with thethreshold level to determine the data signals represent 1's or 0's.

A decoder 60 converts the 1 and 0 indications from the comparing meansinto a digital data train. More specifically, the decoder 60 is clockedby the clock means 46 to produce digital 1's and 0's in accordance withthe 1 or 0 indication from the comparing means.

A difference means 62 determines a difference between the peakamplitudes to produce a tracking error signal. An automatic gain controlmeans 64 adjusts the difference or tracking error signal in accordancewith the threshold signal. A conventional tracking servo circuit 66utilizes the threshold adjusted or tracking error signal forautomatically adjusting the alignment of the optical spot and thevirtual track.

In the preferred embodiment, the recording medium is an optical magneticdisk. As is conventional in the art, data is recorded in a magnetizablefilm of the disk by varying the magnetization in a vertical magnetizingdirection. The polarization plane of the laser beam 20 rotates accordingto the direction of magnetization. The data signal means 56 derives theread data signal from the polarization rotation. The light intensitysignal means 42, by distinction, detects changes in the amount ofreflected light. In this manner, the light intensity signal means isonly able to recognize information from the wobble pits or markers.Again, as the laser spot deviates from the center of the virtual track,the total amount of light reflected from the data region 12, hence themagnitude of the data signal, is reduced.

In a read only type optical disk, the data is commonly encoded in pits.Various prior techniques are known for distinguishing between the datapits and the wobble pits. For example, a series of wobble pits isarranged in a regularly repeating pattern as in sector orsynchronization marks. The pattern or periodicity in the wobble pitsmakes them distinguishable from the more randomly, disposed data pits.As another alternative, a code may be provided in or among the wobblepits as illustrated in greater detail in the above reference U.S. Pat.No. 4,562,564.

Looking to FIG. 2 in greater detail and FIG. 3 in which the outputsignals of selected components are denoted by the same referencenumeral, an invertor 70 amplifies and inverts the reflective signalpeaks forming the output denoted at 70 in FIG. 3. The peak samplingmeans 44 includes a peak hold circuit 72 which resets itself to andholds the highest value or amplitude of the input signals which itreceives until it is reset. A first sample and hold circuit 74 samplesand holds the value of the peak hold circuit 72 corresponding to thefirst optical markers 14. A second sample and hold circuit 76 samplesand holds the output value of the peak hold circuit 72 corresponding tothe second optical markers 16.

A peak position detector 80, more specifically a differentiating circuit81, accurately detects the center of each of peaks 26, 28, hence of thewobble pits. The differentiating circuit may be a simple circuit formedof a resistor and a capacitor or it may be an active differentiatingcircuit. A comparing circuit 82 compares the output 81 of thedifferentiating circuit with a fixed threshold value to produce a binarypulse that marks the center of each peak. The clock circuit 46 includesa phase lock loop 84 which multiplies the binary pulse of signal 82 byan integer N, 12 in the preferred embodiment to produce a clock signal.In this manner, 12 clock pulses are generated between each of the firstand second optical marks 16 and 18 which enables up to 12 bits of datato be stored in the data segment 12. Changing the clock sequence changesthe number of bits of data that are stored in each data sequence. Adivider circuit 86 divides the output 84 of the clock circuit by N, thenumber of clock pulses between each pair of pits to produce an outputsignal which is returned to the phase lock loop circuit. The phase lockloop circuit compares the signal from comparing means 82 with the signalfrom divider 86 and acts as a servo circuit to change the frequency andphase angle of the oscillating output such as the phase angle of the twosignals coincide with each other. In this manner, the clock pulses aremaintained synchronous with the binary pulse from the comparator 82.

The logic circuit means 48 derives control signals for the peak samplingmeans 44 from the clock signal 84. A second divider 90 divides theoutput of the first divider 86 by 2 to produce an output which marksalternate optical markers, i.e. left or right markers. The clock signalfrom the phase clock loop circuit 84 is counted by a counter 92 toproduce parallel, digital output signals which are received by a decoder94. The decoder 94 has a sample output 94a which produces a samplesignal 94a at pre-selected number of counts after each reflective lightintensity signal peak, two counts in the preferred embodiment. Adistribution circuit 96 is controlled by the output signal on the seconddivider 90 to distribute the sample signal between two outputs, 96a and96b. The first output 96a causes the first sample and hold means 74 tosample the output of the peak hold circuit 72, two counts after thecenter of each first optical marker 14. The second output 96b causes thesecond sample and hold 76 to sample the output of the peak hold circuit72, two counts after each second optical marker 16. The decoder 94further produces a reset signal on a reset output 94b a pre-selectedduration after each optical marker to reset the peak hold circuit 72before the laser spot 20 crosses the next marker or pit. In thepreferred embodiment, peak hold circuit is reset 8 counts after thepreceding pit.

The decoder 94 may be a read only memory or may utilize simple gatelogic. The distribution circuit 96 may be a conventional selectorcircuit which is stepped by the high/low state at the output of thesecond divider 90.

The adding means 52 includes a summing junction and an amplifier whichsums the outputs of the first and second sample and hold circuits. Thesum 52 of the peak levels or threshold level does not change widely evenwhen the lightspot deviates from accurate alignment with the track. Theamplitude or gain adjusting means 54 includes a voltage divider 100which reduces the amplitude of the sum from the adding means 52. Theamplitude of the data signal 56 is adjusted by an amplifier 102 of thegain adjusting means 54. In this manner, the gain adjusting means 54adjusts the relative gain or amplitude at the data signal and thethreshold signal. When the voltage divider or the gain adjusting meanseffectively divides the sum by 2, for example, then the sum is actuallyrepresentative of the average of the two sample and hold signals.

The comparing means 58 includes a comparator or differential amplifierwhich receives the threshold signal 100 in one input and the data signal56 in the other. When the data signal exceeds the threshold level, thecomparator 58 produces a high or 1 signal. When the data signal is belowthe threshold value, the comparator produces a low or 0 signal. Thedecoder 60 decodes the 1 and 0 signals into original data words.

The subtraction means 62 includes a differential amplifier whose outputrepresents a difference between the levels of the outputs of the 2sample and hold circuits. The differential signal, which represents thetracking error, is fed to the automatic gain control circuit 64.

The sum signal is conveyed to the automatic gain control 64. Theautomatic gain control circuit adjusts the error signal in accordancewith the variation in the sum or threshold signal.

In the preferred embodiment, the optical detection system is set suchthat the variations in the data signal have the opposite polarity to thewobble pit signals. By changing the polarity of the data and pitsignals, the pit signals are easily separated from the data.

In the preferred embodiment, the data signals are quantized into merely1's or 0's, i.e. a binary quantitization, with a single threshold value.Optionally, the data can be compressed such that each of the datapositions in the data region 12 can store a plurality of bits. Forexample, the voltage divider may produce a plurality of thresholdvalues, for example 3 threshold values. The comparing means may comparethe amplitude of the analog data signal 56 with the 3 thresholds whichdefine four amplitude regions. The comparing means may produce any oneof four outputs, expressed binarily 00, 01, 10, 11. As another option,the data signal or the binary signal may be differentiated and the zerocrossing point may be detected analogously to the detection of thecenter of the pits signals by a differentiating means and a digitizer.

With reference to FIG. 4, as yet another option first and second lowpass filters 110 and 112 may be inserted after the first and secondsample and hold circuits 74 and 76 or after the adding means 52 tosmooth the signals. The low pass filter reduces a variation in the sumsignal 52' from the adding means 52 as illustrated in FIG. 4.

With reference to FIG. 5, the wobble pits may be elongated having ashort length L and a width W. In the embodiment FIG. 5A, the pit width Wis set to half the interval between adjacent tracks. In this manner, thesum of the peak values from the pits on the left and the right sides ofthe tracks will remain substantially unchanged even when the spotdeviates from the track. The pit length L is preferably larger than thediameter of the lightspot to promote a sufficient signal level. Whendifferential detection is used to detect the peak center position, thepit length L may be made smaller than the lightspot diameter.

In the preferred embodiment of FIG. 5B, the pit length L is greater thanthe pit width W. In the embodiment of FIG. 5C, the second pits have beencombined into single pit which enables the tracks to be positioned moreclosely adjacent.

In the preferred embodiment, the pit signals and the data signals arehandled by separate systems. However, there are other methods ofdetecting the optical magnetic signal. In one prior art method, a singleoptical element such as a polarized beam splitter and a Glan-Thompsonanalyzer are utilized to transmit specific polarization components. Inanother prior art method, two optical elements is used to turn thepolarization plane penetrating axis of the optical elements in oppositedirections from the extinction axis, receive the lights that havepenetrated the optical elements by the light detectors, take thedifference, and obtain the magnetized signal. Optionally, a halfwavelength plate may be utilized to turn the polarization plane or thereflected light from the disk by 45 degrees. This functions as apolarized beam splitter to separate the penetrating and reflected light.By taking the difference between the penetrating and reflected light,the data and wobble pit signals can be separated.

The differential detection of the optical magnetic signal is better ableto reduce the noise components that depend on the quantity of light suchas laser noise and disk noise by offsetting them against each other.Accordingly, when there is an interference between the pit signals andthe data signals, the pit components are offset against each other andeliminated because they represent changes in the reflected lightquantity. The magnetization data, by distinction, is indicated bychanges in polarization.

In the embodiment of FIG. 6, the data signal 56' is detected bydifferential detection. It can be seen that the reflected light from thewobble pits does not appear due to the offset. The level of thedifferential signal portions 26', 28' attributable the wobble pits isalmost constant regardless of the density of the data. Accordingly, itis possible to sample and hold the levels of the differential ortracking error signal during periods 26' and 28' and use them as thethreshold reference. If the pit signals are not completely offset by thedifferential detection, the areas immediately before and after thewobble pits, where data is not recorded, may be extended. The signallevel from this unrecorded area is detected relative to the level 26',28' attributable to the pits to remove the undesirable effect of thepits.

The invention has been described with reference to the preferredembodiment. Obviously, modifications and alterations will occur to thoseof ordinary skill in the art on reading and understanding the precedingdetailed description. It is intended that the invention be construed asincluding all such alterations and modifications insofar as they comewithin the scope of the appended claims or the equivalent thereof.

Having thus described the preferred embodiment, the invention is nowclaimed to be:
 1. An apparatus for retrieving recorded data, theapparatus comprising:a recording medium having at least first and secondoptical markers disposed adjacent a recording track along which data isencoded: an optical scanning means for scanning the recording track withan optical beam; a light detecting means for generating a lightintensity signal which changes as the optical beam scans the opticalmarkers; a first sampling means for sampling the light intensity signalas the beam scans adjacent to the first optical markers to produce firstsampled light intensity signals; a second sampling means for samplingthe light intensity signal as the optical beam scans adjacent the secondoptical markers to produce second sampled light intensity signals; acombining means for combining the first and second sampled lightintensity signals; a data signal means for generating an analog datasignal in accordance with the encoded data; and, a comparing means forcomparing the combined first and second sampled light intensity signalswith the analog data signal.
 2. The apparatus as set forth in claim 1wherein the first optical markers include a series of pits at intervalsalong one side of the data track and wherein the second optical markersinclude a series of second pits along another side of the recordingtrack.
 3. The apparatus as set forth in claim 1 further including a peakdetector for detecting a peak signal value corresponding to the changein the light intensity signal as it scans each optical marker.
 4. Theapparatus as set forth in claim 3 wherein the optical markers are spacedwith a regular periodicity and further including a clock meansoperatively connected with the peak detector for deriving a clock signalfrom the changes in the optical beam as it scans the optical markers. 5.The apparatus as set forth in claim 4 further including a logic meansoperatively connected with the clock means for receiving clock signalstherefrom and operatively connected with the peak detector and the firstand second sampling means for controlling sampling and resettingthereof.
 6. The apparatus as set forth in claim 1 further including anamplitude adjusting means for adjusting the relative amplitude of thecombined first and second sampled light intensity signals and the datasignal.
 7. The apparatus as set forth in claim 6 wherein the comparingmeans compares the analog data signal with the combined first and secondsampled light intensity signals and produces 1 and 0 indications inaccordance with the comparison, whereby the 1 and 0 indications aredetermined in accordance with an alignment of the optical beam and therecording track.
 8. The apparatus as set forth in claim 7 wherein thecombining means includes an adding means for summing the first andsecond sampled tracking signals, whereby the analog data signal iscompared with a sum or average of the first and second light intensitysignals.
 9. The apparatus as set forth in claim 1 further including:adifference means for subtractively combining the first and secondsampled light intensity signals to produce a difference signal; anautomatic gain control means for adjusting the difference signal inaccordance with the combined first and second sampled light intensitysignals; and, a servo means for adjusting an alignment between theoptical beam and the recording track in accordance with the adjusteddifference signal.
 10. An apparatus for retrieving data encoded on arecording track that is scanned by a data reading means, the apparatuscomprising:a misalignment monitoring means for monitoring misalignmentbetween the recording track and the data reading means; a thresholdmeans for setting a threshold level, the threshold means beingoperatively connected with the misalignment means for setting thethreshold in accordance with the monitored misalignment; and, acomparing means for comparing the threshold level with data read fromthe recording track, the comparing means being operatively connectedwith the threshold means and the data reading means.
 11. A method ofretrieving data encoded on a recording track which is scanned by a datareading means, the method comprising:reading encoded data from therecording track, monitoring a degree of misalignment between therecording track and the reading means; adjusting a threshold level inaccordance with the degree of misalignment; and, interpreting the readencoded data, in accordance with the threshold level, whereby theinterpretation of read data is adjusted in accordance with a thresholdlevel that varies with the misalignment of the recording track and thedata reading means.
 12. The method as set forth in claim 11 furtherincluding adjusting alignment of the reading means and the recordingtrack in accordance with the monitored degree of misalignment.
 13. Themethod as set forth in claim 11 further including digitizing the readencoded data in accordance with a comparison of the read encoded dataand the threshold level.
 14. The method as set forth in claim 11 whereinthe monitoring step includes monitoring variations in reflected lightcaused by optical markers disposed adjacent to the recording track as alaser spot is scanned along the track.
 15. The method as set forth inclaim 14 wherein the reading step includes monitoring variations inpolarization of reflected light as the laser spot is scanned along atrack.
 16. A method of retrieving encoded data from a recording trackwhich is scanned by an optical beam and which has first and secondoptically recognizable referenced markers therealong, the methodcomprising:scanning the optical beam along the track, monitoring opticalvariations as the beam is scanned adjacent the first and second opticalmarkers., adjusting a threshold level in accordance with the monitoredoptical variations; reading encoded data from the track; and, comparingthe read encoded data with the threshold level.
 17. The method as setforth in claim 16 wherein the first optical markers are disposedadjacent one side of the track and the second optical markers aredisposed adjacent an opposite side of the track and wherein the step ofmonitoring optical variations includes:deriving a first light intensitysignal in accordance with the optical variations attributable to thefirst optical markers; deriving a second light intensity signal inaccordance with optical variations attributable to the second opticalmarkers; and, combining the first and second light intensity signals,the threshold level being adjusted in accordance with the combined lightintensity signals.
 18. The method as set forth in claim 17 wherein thestep of combining the light intensity signals includes summing the firstand second light intensity signals.
 19. The method as set forth in claim18 further including:subtracting the first and second light intensitysignals to create a tracking error signal; adjusting the tracking errorsignal in accordance with the sum of the first and second lightintensity signals; and, adjusting alignment of the optical beam and thetrack in accordance with the adjusted tracking error signal.